Drag coefficient and its sea state dependence under tropical cyclones

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  • 1 a Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USA
  • | 2 b Applied Physics Laboratory and School of Oceanography, University of Washington, Seattle, WA, USA
  • | 3 c Institute of Oceanography, National Taiwan University, Taipei, Taiwan
  • | 4 d NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA
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Abstract

The drag coefficient under tropical cyclones and its dependence on sea states are investigated by combining upper ocean current observations (using EM-APEX floats deployed under five tropical cyclones) and a coupled ocean-wave (Modular Ocean Model 6 - WAVEWATCH III) model. The estimated drag coefficient averaged over all storms is around 2−3×10−3 for wind speeds 25–55 m/s. While the drag coefficient weakly depends on wind speed in this wind speed range, it shows stronger dependence on sea states. In particular, it is significantly reduced when the misalignment angle between the dominant wave direction and the wind direction exceeds about 45°, a feature which is underestimated by current models of sea state dependent drag coefficient. Since the misaligned swell is more common in the far front and in the left front quadrant of the storm (in the Northern Hemisphere), the drag coefficient also tends to be lower in these areas and shows a distinct spatial distribution. Our results therefore support ongoing efforts to develop and implement sea state dependent parameterizations of the drag coefficient in tropical cyclone conditions.

Corresponding author: Xiaohui Zhou, xiaohui_zhou@uri.edu

Abstract

The drag coefficient under tropical cyclones and its dependence on sea states are investigated by combining upper ocean current observations (using EM-APEX floats deployed under five tropical cyclones) and a coupled ocean-wave (Modular Ocean Model 6 - WAVEWATCH III) model. The estimated drag coefficient averaged over all storms is around 2−3×10−3 for wind speeds 25–55 m/s. While the drag coefficient weakly depends on wind speed in this wind speed range, it shows stronger dependence on sea states. In particular, it is significantly reduced when the misalignment angle between the dominant wave direction and the wind direction exceeds about 45°, a feature which is underestimated by current models of sea state dependent drag coefficient. Since the misaligned swell is more common in the far front and in the left front quadrant of the storm (in the Northern Hemisphere), the drag coefficient also tends to be lower in these areas and shows a distinct spatial distribution. Our results therefore support ongoing efforts to develop and implement sea state dependent parameterizations of the drag coefficient in tropical cyclone conditions.

Corresponding author: Xiaohui Zhou, xiaohui_zhou@uri.edu
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